Dc motors deliver higher performance and efficiency

21st February 2013

Dc motors are increasing in popularity due to their performance advantages over ac motors for applications ranging from high-speed automation to electric motorbikes. Jon Severn reports on a selection of recent developments in dc motors and drives.

Three-phase ac induction motors are the most common type of motor used in industrial application, due largely to their simple design, low purchase cost, reliability and broad choice of sizes and specifications. However, for applications where speed needs to be varied or torque needs to be controlled with high accuracy, brushed dc motors are popular. Where even higher performance and/or reliability are required, brushless dc motors overcome the drawbacks often associated with brushed dc motors.

As well as industrial applications, brushed and brushless dc motors are also employed in smaller machines and equipment where speed control is critical - such as computer hard disk drives - or in battery-powered, automotive and other applications where a dc supply is readily available. It should also be remembered that brushless dc motors are non-sparking and therefore do not generate ozone, which makes them suitable for use in medical applications. Compared with brushed dc motors, brushless types have the further advantage of producing less electrical noise.

Today there are several factors behind the rising demand for dc motors. For example, cars are being offered with more powered devices, plus designers are seeking to improve the performance of the equipment they design, which often entails upgrading from an ac motor to a dc motor. In particular, coreless (ironless) dc motors have low inertia so are useful where high accelerations are required. Interest in alternative energy sources often results in a high-efficiency dc motor being used for the final drive.

For example, the prototype ENV motorbike from Intelligent Energy features a fuel cell that generates electrical energy from hydrogen and oxygen. To make the best use of the limited supply of electrical energy, the bike is equipped with a high-efficiency Lynch axial-gap dc motor. This type of low-voltage, high-torque, permanent-magnet dc motor has the added advantage of a high power density. Lynch motors are today manufactured in the UK by the Lynch Motor Company in three frame sizes and with numerous options. Alternatively two motors can be coupled together to drive a single output shaft, thereby increasing the power available. To illustrate the motors' capability, the LEM 130 model 95s has a rated power of 3.02kW, rated torque of 4.35Nm and rated speed of 6624rpm. Peak efficiency is 87 per cent. For more demanding applications, the LEM 200 model D135 has a rated power of 14.39kW, rated torque of 36.4Nm and rated speed of 3780rpm. Peak efficiency for this model is 90 per cent.

Motorcycle applications

Cedric Lynch, inventor of the Lynch motor, now works with Agni Motors in India. Here he has further improved the design for applications in the domestic, leisure, industrial, commercial, agricultural and marine industries. The latest version of the Agni Motor is said to offer higher torque per amp and lower speed per volt, which makes it possible to use a simpler and cheaper transmission in some applications, as well as being slightly more efficient. Two Agni motors are being installed in a Ducati 600 Supersport, which is intended to be the fastest, road-legal, emission-free motorcycle in Europe. Power will be supplied by a LiFePO4 (lithium iron phosphate) battery developed by Lifebatt. The bike will be entered in the TTxGP clean-emissions race, which takes place on 12 June 2009 on the Isle of Man TT circuit, one of the most gruelling road-race circuits in the world.

In the USA, Briggs & Stratton has licensed Lynch motor technology to manufacture the Etek motor for use primarily in electric outboard motors for boats. Once again, however, this type of motor has also been selected for use in motorbikes. This time the company is Electric Motorsport of the USA, with its Electric Motorsport GPR-S using 3.3kWh Lithium batteries rather than the fuel cell of the ENV. The GPR-S is claimed to have a maximum speed of 112km/h (70mph) and a range of 80km (50miles). While the bike is equipped with the Etek-RT motor, the company says that the industry-standard motor mounting would make it straightforward to retrofit a different motor (and controller) to take advantage of regenerative braking.

An alternative to the Lynch, Agni and Etek motors is the PMG 132 high-performance disc armature motors from Perm Motor in Germany, which operate from 24-72V and have a rated power output of 2.2 to 7.2kW. Depending on the operating conditions, these can achieve a peak efficiency of 90 per cent or more.

In-wheel motors

PML Flightlink has been manufacturing printed armature motors (or 'pancake' motors) for some 40 years. In this type of motor, the flat armature is essentially an ironless disc with coils punched from sheet copper and formed onto non-magnetic disc insulators. Brushes contact on two or more points to create a path through the copper, thereby creating a magnetic field that interacts with the permanent magnets to cause motion.

With no magnetic material present in the armature, 'cogging' is eliminated, resulting in smooth operation and continuous torque down to standstill, with minimal torque ripple. Torque is proportional to current and not limited by saturation, and speed is directly proportional to voltage.

Furthermore, the ironless armature has virtually zero inductance, which is beneficial in that when the magnetic field collapses, energy does not discharge by arcing to the brushes. Consequently the printed armature exhibits low EMC properties and brush wear is minimal. Indeed, PML claims to have achieved over 10000 hours working life on a high-volume product.

Recent developments from PML Flightlink include the Hi-Pa Drive in-wheel unit that incorporates the motor and drive electronics within a common housing for automotive drive applications. This has already been applied to the Mini QED and the Volvo ReCharge. Scaling this concept down, the company has also created the Nano Wheel that incorporates a brushless motor within a wheel hub.

In addition, the wheel utilises patented integral suspension that is injection moulded in the same operation as the hub. The first application for this unit is a power-assisted suitcase, and a powered golf trolley is currently in development. Compared with conventional motor technologies, the Nano Wheel is claimed to offer a more compact, simpler mechanical design, with fewer moving parts and less friction. Products that fold for storage or transit are therefore easier to design, and the energy efficiency and low friction help to maximise battery life.

Compact dc motors are also being manufactured in ever-increasing volumes for even smaller applications. These include solar-powered devices, toys and mobile telephone handsets - for the vibration function.

Apart from those systems in which the dc motor is connected directly to a switched power supply, the motor is only one half of the dc powertrain. Industrial applications and those calling for precise control of speed or torque require a dc drive. Given the increased demand for dc motors, it is not surprising that there have also been developments recently in the field of dc drive technology.

One of the companies most active in this area is Sprint Electric. This company's 340XRi, 680XRi and 1220XRi dc four-quadrant dc drives, for motors up to 1.8kW (2HP), are compact and energy-efficient, being designed to regenerate energy back into the mains ac supply under braking without the need for intermediate energy storage, resistive dumping or additional power bridges.

At the 2008 SPS/IPC/Drives show in Nuremberg, Germany, Sprint unveiled two- and four-quadrant PLX digital drives with current ratings up to 1650A, which is a major step up from the previous maximum rating of 1050A.

Another dc drive launched at Nuremberg was the Mentor MP. Control Techniques claims this drive sets a new world standard for excellence in dc motor control, and also that it is the only RoHS-compliant dc systems drive on the market.

Control Techniques has a patent (pending) to protect an innovative aspect of the Mentor MP's design. Galvanic isolation between power and control is a standard feature in ac drives and, in the case of a power-side failure, it protects the control circuits and connected equipment. According to Control Techniques, other dc drives feature a form of isolation that relies on impedance-only isolation, which has limitations. In contrast, the Mentor MP uses novel technology to achieve galvanic isolation without compromising performance or reliability.

Each Mentor MP has three universal option module slots in which customers can install any combination of the 18 modules available. These economically priced modules are common across Control Techniques' ac, dc and servo drive ranges, offering functions such as feedback options, expanded I/O and programmable control. Furthermore, the range of optional fieldbus connectivity is extensive, including Profibus-DP, Interbus-S, Devicenet, Canopen and various Ethernet-based protocols such as Modbus TCP/IP, Ethernet/IP and Ethercat.

Parker Hannifin's Parker SSD Drives division has also introduced new dc drives, such as the Frame 6 version of its DC590+ family. This is much smaller than previous models, with the company claiming that it requires less than 50 per cent of the cabinet space compared with other dc drives of similar current ratings. The DC590+ Frame 6 high-power drive is available in a choice of three current ratings of 1250, 1600 and 1950 A, with voltage ranges from 380 to 690V ac and power ranges from 600 to 900kW (750 to 1250HP).

According to Parker SSD Drives, the DC590+ Frame6 dc drive has been designed to provide optimum reliability and efficiency with a range of features to minimise wear and save energy. For example, cooling fans are only switched on when required and are constantly monitored for rotation, and built-in snubbers ensure reliable operation even in the event of mains disturbance.

To improve operational flexibility still further, option cards enable the new drive to communicate with popular industrial protocols, including Ethernet, Profibus, Controlnet, Devicenet and Canopen. The DC590+ Frame6 is also supplied with a full set of function blocks dedicated to section control, including winder/unwinder, dancer (tension) control, PID and load control. In some applications this eliminates the need for a separate logic controller.

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